Process for the production of portland type cement clinker

ABSTRACT

An improved process for producing portland and portland type cement clinker wherein argillaceous and calcareous type materials are ground and blended to form a generally homogeneous mixture and subsequently burned in a kiln at a temperature in the range of about 1200° C. to about 1500° C. to form cement clinker, the improvement comprising incorporating into the mixture a fluorine containing mineral acid, selected from the group comprising fluotitanic acid, fluoboric acid, fluophosphoric acid, fluosilicic acid, and hydrofluoric acid, as a fluxing and mineralizing agent, in an amount of up to 3.0% by weight based upon the weight of the dry solids in the mixture.

BACKGROUND OF THE INVENTION

This invention relates generally to processes for producing portland andportland type cement clinker, and more particularly to clinkeringprocesses wherein a fluxing and mineralizing agent is incorporatedtherein.

Portland cement clinker is typically formed by burning an argillaceoustype material such as clay, and a calcareous type material such aslimestone, in a kiln at a temperature of about 1450° C. for a period oftime, generally in excess of one hour, such that the raw materials reactto form new compounds which, when mixed with water, will hydrate and setup as a hard mass. In this reaction, the calcareous or limestonematerial, e.g., CaCO₃, in the presence of heat gives off CO₂ and becomesCaO or free lime, which then reacts with the silicon dioxide (SiO₂),aluminum oxide (Al₂ O₃), and ferric oxide (Fe₂ O₃) which are thepredominant components of argillaceous or clay type materials. Theresult is that the free lime (CaO) is consumed and reacted and convertedto tricalcium silicate or alite, which is the principal component ofportland cement clinker. Other calcium-silicate compositions, such asdicalcium silicate, calcium-aluminate compositions, such as tricalciumaluminate (3CaO . Al₂ O₃), and calcium-aluminum-iron compositions and/orsolid solutions, such as tetracalcium aluminoferrite (4CaO . Al₂ O₃. Fe₂O₃), are formed in the reaction and these constitute minor proportionsof the resulting clinker composition. A usual, commercial portlandcement clinker will contain less than 2.0% by weight free lime,frequently less than 1.0% by weight, and also will contain as much as55% by weight alite, frequently more than 60% by weight alite.

Portland cement is defined in ASTM C150-74 as a hydraulic cementproduced by pulverizing clinker consisting essentially of hydrauliccalcium silicates, usually containing one or more forms of calciumsulfate as an interground addition. The chemical requirements specifyneither a maximum amount of free lime (CaO) nor a minimum amount ofalite, i.e., tricalcium silicate (3CaO . SiO₂). The physicalrequirements are among others, that mortars, the testing of which isspecified in ASTM C109-73, made using ASTM Type I portland cement,achieve 3 and 7 day compressive strengths of not less than 1800 psi(pounds per square inch) and 2800 psi, respectively.

In the clinkering process, it is known in the art to add substances inaddition to the raw materials, which substances aid in the cementburning, e.g., in the kiln, and these substances are known as fluxingand/or mineralizing agents. As discussed in Lea, The Chemistry of Cementand Concrete, 3rd Edition, 1971, Chemical Publishing Company, on pages156 and 157, these agents lower the temperature at which liquid or meltis formed (in the burning process) and thus the clinkering temperature.Examples given in Lea are calcium fluoride (CaF₂) and the fluosilicates,such as sodium, magnesium and calcium fluosilicate.

Normally, a flux is a substance which decreases the melting point of theliquid phase, while a mineralizer is a substance which increases therate of a process and/or reaction occurring within the solid phase, theliquid phase or at the liquid-solid interface. Fluorspar (CaF₂)functions as both a flux and a mineralizer; it lowers the temperature atwhich liquid is formed, thus reducing the clinkering temperature; and italso increases the reactivity of free lime with intermediary clinkermaterials.

In addition to lowering the clinkering temperature, fluxes have beenused to facilitate clinkering. For example, in 1887, LeChatelier usedboth calcium chloride and calcium fluoride in his attempts to synthesizepure tricalcium silicate. Further the burnability ofdifficult-to-clinker mixes, such as low liquid phase compositions, e.g.,white portland cement, which is more difficult to burn because of theabsence of the iron compounds which partly form the liquid phase, isenhanced using fluxes, such as cryolite (Na₃ AlF₆) which has been usedto aid in burning white portland cement clinker where iron contaminationis to be avoided.

With the growing scarcity and unavailability of energy, there is anincreasing need for new and/or alternative ways of saving and reducingthe amount of energy needed for producing cement. For example asreported in Energy Conservation Potential in the Cement Industry, areport by the Portland Cement Association for the Federal EnergyAdministration, published as Conservation Paper No. 26, June, 1975,today in the U.S., a typical cement kiln is operated at a temperature ofapproximately 1450° C., and averages about 1600 kcal/kg of cementproduced, and so even a modest reduction (100° C.) in the kiln burningtemperature could result in energy savings on the order of 150 kcal/kgof clinker. Since fluxing and mineralizing agents reduce the clinkeringtemperature, there is a corresponding need for improved and/oralternative fluxing and/or mineralizing agents.

SUMMARY OF THE INVENTION

The present invention is directed generally to a process for producingportland type cement clinker, and more specifically to an improvedprocess for producing portland type cement clinker using a fluxingand/or mineralizing agent.

The present invention has resulted from the unexpected discovery thatcertain fluorine containing inorganic or mineral acids provideunexpected levels of fluxing and mineralizing in manufacturing portlandcement clinker and result in greater energy savings than do the knownprior art fluxing and mineralizing agents.

For the purposes of this application, the terms fluorine containingmineral acid and fluorine containing inorganic acid are usedinterchangeably and are intended to mean acids comprised of inorganicelements and containing fluorine as a constituent. The fluorinecontaining mineral or inorganic acid can be, for example, fluotitanicacid or hexafluorotitanic acid or dihydrogen hexafluorotitanate (H₂TiF₆), fluoboric acid or tetrafluoroboric acid (HBF₄), fluophosphoricacid, such as fluorophosphoric acid or phosphorofluoridic acid (H₂ PO₃F), difluorophosphoric acid or phosphorodifluoridic acid (HPO₂ F₂) andhexafluorophosphoric acid or hydrogen hexafluorophosphate (HPF₆),fluosilicic acid or hexafluorosilicic acid or dihydrogenhexafluorosilicate (H₂ SiF₆) and hydrofluoric acid (HF). Fluotitanic andfluosilicic acids are the preferred fluorine containing mineral acids.

The fluorine containing mineral acid is employed to flux and/ormineralize portland or portland type cement raw material in an amount ofup to 3.0% by weight based upon the weight of the dry solids in the rawmaterial to be burned and converted into clinker. A preferred additionis in an amount in the range of from about 0.1% by weight to about 1.0%by weight, with the range 0.2% by weight to 0.5% by weight being anadditional preferred range.

For the purposes of this invention, the term portland type cement isintended to include all cementitious compositions which are portlandcement, or are chemically similar or analogous to portland cement, thespecification for portland cements being set forth in ASTM C150-76a.This would include white cements, expansive portland cement, regulatedset cements, as well as cements in which fly ash, such as from steam orpower generating stations, pozzolana slag, such as from blast furnaces,alkali waste, such as precipitated calcium carbonate obtained as aby-product in the alkali and synthetic ammonium sulphate industries, ormixtures of these are incorporated in and result in a portland typecement. Thus, a wide variety of argillaceous and calcareous type rawmaterials can be employed to make portland type cement clinker andultimately portland type cement.

Basically, the process comprises incorporating the fluorine containinginorganic acid into the raw materials to be burned either by adding itto dried raw material which then passes directly to the kiln or byadding it to raw material which is in the form of a slurry and which issubjected to a "wet" process burning or undergoes a subsequent dryingstep prior to the burning step. By either method, the burning step isrendered easier and requires less energy to convert the raw materialinto portland type cement clinker.

It is therefore an object of the present invention to provide animproved process for producing portland and portland type cementclinker.

These and other objects and advantages of the invention will becomeapparent on consideration of the description and discussion whichfollows.

DETAILED DESCRIPTION OF THE INVENTION

As is old and well known in the cement processes of the art, the kilnfeed may be processed to portland type cement clinker by either the"dry" process or the "wet" process.

Typically in a "dry" process, the admixture of argillaceous andcalcareous type materials is proportioned, ground and blended. The mixedmaterials then pass through raw grinding mills, in which the fineness isbrought to about 200 mesh. The raw materials are then passed to thecement kiln and burned in the usual manner to a portland type cementclinker.

In a typical "wet" process, the admixture of components is proportioned,made into a slurry with water and ground to a uniform fineness,approximately 200 mesh (Tyler), the water content of the slurry usuallybeing between 30% and 40%. This slurry is fed to the cement kiln andburned to a portland type cement clinker, the water content of theslurry being volatilized and carried over as steam with the kiln gases.

No attempt will be made in the present instance to describe in anydetail the processes involved in drying, proportioning, grinding andpreparing the raw materials required to the kiln feed, the burning ofthe mix to clinker or the subsequent treatment of the clinker in themanufacture of the various types of portland cement. All of this is wellknown to persons skilled in the art.

If the "wet" process is employed in the process of this invention, thefluorine containing inorganic acid can be added as a concentrate or as amore dilute solution along with the water employed to make the kiln feedslurry. If the "dry" process is employed, a more concentrated form ofthe acid may be used.

What is critical to the present invention is that the flux be a fluorinecontaining inorganic acid, and be included in certain amounts. Thefluorine containing inorganic or mineral acid can, for example, comprisefluotitanic or hexafluorotitanic acid (H₂ TiF₆), fluoboric ortetrafluoroboric acid (HBF₄), fluophosphoric acids, such asfluorophosphoric or phosphorofluoridic acid (H₂ PO₃ F),difluorophosphoric or phosphorodifluoridic acid (HPO₂ F₂) andhexafluorophosphoric acid (HPF₆), fluosilicic or hexafluorosilicic acid(H₂ SiF₆) and hydrofluoric acid or hydrogen fluoride (HF), withfluosilicic and fluotitanic acids being preferred. Hydrogen fluorideusually occurs as a gas, as compared to hydrofluoric acid which is awater solution of hydrogen fluoride. Although the gaseous form is lesspreferred than the solution form of HF in view of the handling problems,etc., it is nonetheless considered to be within the scope of the presentinvention.

The fluorine containing inorganic acid is employed in an amount of up to3.0% by weight based upon the weight of the dry solids in the rawmaterial to be burned and converted into clinker, preferably in anamount in the range of from about 0.1% to 1.0% by weight with the rangeof from 0.2% to 0.5% by weight being also preferred, and can be used toflux and/or mineralize any portland or portland type cement rawmaterial.

The fluorine containing acids to be employed can be pure reagent gradeacids or industrial grade acids, such as those which are the by-productof other chemical reactions. For example, fluosilicic acid is aby-product of phosphoric acid manufacture and is produced as an aqueoussolution at a concentration which usually varies between 23% by weightand 25% by weight. Industrial grade fluosilicic acid solution may alsocontain about 0.1% by weight P₂ O₅ and about 15 ppm of heavy metals.Further in the industrial grade solution, a slight excess of free HF maybe present in the solution to stabilize it and prevent any silicaprecipitation.

To give those skilled in the art a better understanding of theinvention, a number of examples were run. The examples are offeredmerely by way of illustration, and it is not intended that they be takenas limiting the scope of the invention. In the examples, three raw feedswere used, and the typical analyses of these feeds, as well as theirpredicted, potential clinker mineral compositions, which have beencalculated from their analyses using the Bogue equations as is known inthe art, are set forth in the following Table I. A discussion of theBogue equations and the method of calculating the potential cementcompounds is set forth on pages 114-117 of Lea, The Chemistry of Cementand Concrete, Third Edition, 1971, Chemical Publishing Co. The feedswere industrial feeds, i.e., were obtained from the feed end of kilnsproducing commercially available cements, and these are hereinafteridentified as raw feeds, A, B and C. Raw feeds A and B are Type Iportland cement raw feeds with raw feed A being produced for use in awet process and raw feed B being produced for use in a dry process,while feed C is a white portland cement raw feed, which is normally moredifficult to burn and is produced for use in a dry process.

                  Table 1                                                         ______________________________________                                        Kiln Feed Analyses                                                            ______________________________________                                        Actual Composition (Percent by Weight)                                        Ingredient  Raw Feed A Raw Feed B Raw Feed C                                  ______________________________________                                        SiO.sub.2   13.88      13.87      15.23                                       Al.sub.2 O.sub.3                                                                          3.27       3.99       2.99                                        Fe.sub.2 O.sub.3                                                                          1.42       1.74       0.54                                        CaO         42.67      42.49      44.88                                       MgO         2.03       0.95       0.92                                        SO.sub.3    0.70       2.05       0.15                                        Na.sub.2 O  0.16       0.2(Est.)  0.2(Est.)                                   K.sub.2 O   0.67       0.65       0.36                                        LOI (Loss on                                                                              35.52      34.45      35.39                                       Ignition at                                                                   950° C.)                                                               Potential Cement Compounds                                                    (Percent by Weight)                                                           C.sub.3 S   65.2       49.2       70.0                                        C.sub.2 S   12.3       23.2       14.1                                        C.sub.3 A   9.7        11.6       10.7                                        C.sub.4 AF  6.7        8.0        2.5                                         ______________________________________                                    

EXAMPLE I

The raw feeds A, B and C were used as received, except that raw feed Awas screened to remove all particles larger than 50 mesh (Tyler). Thisremoved about 3% by weight of the total feed, but did not significantlychange the composition of the feed. Next, an amount of about 1.0% byweight of H₂ SiF₆, in the form of commercially available fluosilicicacid solution, which is the by-product of phosphoric acid manufactureand contains about 24% by weight H₂ SiF₆, about 0.1% by weight P₂ O₅ and15 ppm of heavy metals (as lead), was added and thoroughly blended witheach of the feeds which were in a dry form. After blending, the mixtureswere clinkered, along with control samples which contained no flux, byheating them in platinum dishes at 1200° C. for periods of 0.5 hours,1.0 hour, and 2.0 hours. After clinkering, the quantity of unreactedlime (CaO), also known as free lime, remaining after heating wasdetermined by X-ray diffraction analysis.

In X-ray diffraction analysis, a sample is exposed to a beam of X-rays,and the diffraction pattern is used to identify the particular crystalstructure and hence composition of the crystalline solid. For thepurpose of the X-ray diffraction analysis tests involving thisapplication, the CaO peak was measured at 37.3° 2θ (theta). Day-to-dayvariations in X-ray tube intensity were corrected by also running anovaculite (alpha-quartz) intensity standard, and then normalizing alldata by the daily novaculite intensity ratio. Free lime content wasobtained from a calibration curve constructed from samples which wereindependently analyzed by a wet chemical method given in ASTM C-114.

The results, which are set forth in Table II, demonstrate the effect ofthe flux in the clinkering process. The raw feed C contained arelatively high level of free lime after burning, but nonetheless theuse of a fluorine containing acid reduced that level of free lime anddomonstrated the effect of such acids in a mix which is very difficultto burn.

                  Table II                                                        ______________________________________                                        Free Lime Content of Raw Feed Samples Containing 1.0%                         By Weight H.sub.2 SiF.sub.6 and Burned at 1200° C. for Various         Times                                                                                             Free Lime (%)                                                      Time     Temperature         1.0%                                    Raw Feed (Hr)     (° C.)                                                                             Control H.sub.2 SiF.sub.6                       ______________________________________                                        A        0.5      1200        17.3    5.6                                              1        1200        16.6    2.8                                              2        1200        16.4    1.6                                     B        0.5      1200        22.5    4.4                                              1        1200        21.3    1.9                                              2        1200        20.8    1.2                                     C        0.5      1200        56.4    24.9                                             1        1200        51.2    22.4                                             2        1200        49.8    17.8                                    ______________________________________                                    

EXAMPLE II

Varying amounts of fluosilicic acid (H₂ SiF₆) in the form of thefluosilicic acid solution of Example I, were added to wet and dry rawfeed A. In this example, the oversized particles in the feed, which areretained on a 50 mesh screen (Tyler) and had been screened out in theprevious example, were ground and blended back into the screened bulkmaterial. The raw feed which was in a dried condition was divided intotwo portions. One portion was designated a dry feed, and the otherportion was reconstituted into an about 30% by weight water slurry bythe addition of water to form a wet or slurry feed. With the dry feed,the flux was blended into the dry feed, and then the mixture was burned.With the wet feed, the resulting mixture was dried prior to burning (orclinkering), by heating the mixture to 105° C. for several hours untilit had a dry appearance. While the wet feed examples are not the same asthose in a wet burning process, there are a number of similarities. Ineach case the burning was, along with a control sample which containedno flux, for 1 hour at 1300° C. in platinum dishes, and the results areshown in Table III which follows:

                  Table III                                                       ______________________________________                                        Free Lime and Alite Contents                                                  of Raw Feed A Containing                                                      H.sub.2 SiF.sub.6 Mixed With Wet or Dry Feed,                                 and Burned 1 hour at 1300° C.                                          H.sub.2 SiF.sub.6                                                             Addition   Feed      Free CaO    Alite                                        (% by weight)                                                                            Condition (% by weight)                                                                             (% by weight)                                ______________________________________                                        0          wet       8.6         35                                           0.125      wet       5.6         47                                           0.25       wet       2.3         58                                           0.50       wet       1.5         58                                           0.25       dry       4.2         48                                           0.50       dry       2.3         52                                           ______________________________________                                    

Both the free lime and alite (tricalcium silicate) contents wereobtained by X-ray diffraction. For this purpose, the alite peakoccurring at 51.8° 2θ was measured. The alite content representscomparative estimates based upon interpolations from the Boguecalculated value of potential alite content from a well-burned clinkerobtained using raw feed A, which did not contain flux. The results showthat the fluorine containing acid can be added to either a wet or a dryfeed. Further, an apparently more thorough mixing of the flux and rawfeed is achieved with a wet feed since higher alite and lower free limevalues were achieved.

EXAMPLE III

Various amounts of H₂ SiF₆, in the form of the fluosilicic acid solutionof Example I, as well as comparative additions of various amounts ofpowdered CaF₂, a conventional fluxing agent, were added to thereconstituted (about 30% moisture content) raw feed A slurry of ExampleII. After drying, the mixtures, as well as control samples whichcontained no flux, were burned for 1 hour at 1200° C., 1250° C., 1300°C. and 1350° C. in platinum dishes. The results which were obtained byX-ray diffraction and which are set forth in Table IV (all percentagesare by weight), show that the use of the fluorine containing inorganicacid flux, namely fluosilicic acid solution, unexpectedly results in amore thorough burning of the raw feed than does the use of CaF₂ flux,and further that each of these is better than not using any flux, forall of the temperatures involved.

                  Table IV                                                        ______________________________________                                        Free Lime and Alite Contents of Raw Feed A With The                           Flux Added to a Slurry and Burned 1 Hour at Various                           Temperatures                                                                  ______________________________________                                        Amount       Free CaO(%)                                                      Flux    (%)      1200° C.                                                                        1250° C.                                                                      1300° C.                                                                      1350° C.                       ______________________________________                                        None   0         21.3     15.2   8.0    5.9                                   H.sub.2 SiF.sub.6                                                                    0.125     15.6     11.1   5.2    3.6                                   "      0.25      11.4     7.7    2.5    2.1                                   "      0.50      5.6      3.5    1.3    0.7                                   CaF.sub.2                                                                            0.25      15.7     12.3   5.9    5.8                                   "      0.50      12.5     9.4    3.0    2.5                                              Alite (%)                                                                           1200° C.                                                                        1250° C.                                                                      1300° C.                                                                      1350° C.                       ______________________________________                                        None   0          3       19     42     59                                    H.sub.2 SiF.sub.6                                                                    0.125     21       29     50     60                                    "      0.25      31       36     58     58                                    "      0.50      41       48     60     60                                    CaF.sub.2                                                                            0.25      24       34     56     57                                    "      0.50      29       34     54     62                                    ______________________________________                                    

EXAMPLE IV

A dry sample of 1800 grams of as received raw feed A (i.e., includingall oversize particles), was thoroughly mixed and blended with 0.5% byweight (based upon the weight of the dry solids in the raw feed) of H₂SiF₆, in the form of the fluosilicic acid solution of Example I. The mixwas then divided into six equal parts, placed in platinum dishes, andburned for 2 hours at 1300° C. The so formed clinker was reblended andthe complete oxide composition and free lime contents were determined bythe wet method of ASTM C114-69, as well as the free lime and alitecontents, by X-ray diffraction analysis. In addition, the potentialclinker compositions were calculated from the oxide composition usingthe Bogue equations. The results are shown in Table V. Next, the clinkerwas ground in a ceramic ball mill to a Blaine fineness (ASTM C204-73 airpermeability method) of 3450 cm² /g. and blended with 0.0%, 3.0%, and5.0% gypsum (Ca₂ SO₄ . 2H₂ O). The cements hydrated in a normal manner,and the clinker containing 5% gypsum (an amount which is usually foundin portland cements) was subject to a modified Vicat setting timedetermination and indicated an initial set at 31/2 hours and a final setat 7 hours. The procedure was modified in order to accommodate for thefact that the amount of cement produced was not in a sufficient quantityto be subjected to the normal Vicat test. In addition, the clinkercontaining 5% gypsum was used to make nine 2-inch (50-mm) cement mortarcubes having a cement to sand ratio of 1:2.75, a water cement ratio of0.485 and being in accordance with ASTM C-109-73. The resultingcompressive strengths are set forth in Table VI in pounds per squareinch, with these values in megapascals (MPa) being givenparenthetically. As can be seen, mortars made with cement clinkersfluxed in accordance with the teachings of the present invention exceedthe compressive strengths set forth in the ASTM requirements forportland cement (ASTM C150-76a).

                  Table V                                                         ______________________________________                                        Analysis of Fluxed Clinker for                                                Raw Feed A Containing 0.5% by Weight Flux                                     ______________________________________                                        Actual Composition (% by Weight)                                              Ingredient                                                                             Wet Analysis                                                                              X-Ray Diffraction Analysis                               ______________________________________                                        SiO.sub.2                                                                              21.55                                                                Al.sub.2 O.sub.3                                                                       5.25                                                                 Fe.sub.2 O.sub.3                                                                       2.05                                                                 FeO      nil                                                                  CaO      65.30                                                                MgO      3.20                                                                 SO.sub.3 0.65                                                                 Sulfide S                                                                              nil                                                                  Na.sub.2 O                                                                             0.20                                                                 K.sub.2 O                                                                              0.40                                                                 Free CaO 1.45        2.1                                                      Potential Composition (% by Weight)                                           C.sub.3 S                                                                              62.0         60                                                      C.sub.2 S                                                                              15.0                                                                 C.sub.3 A                                                                              10.4                                                                 C.sub.4 AF                                                                              6.2                                                                 ______________________________________                                    

                  Table VI                                                        ______________________________________                                        Test Time  Compressive Strength                                                                           Standard Deviation                                (days)     psi (MPa)        psi (MPa)                                         ______________________________________                                        1          915      (6.31)      ±7 (0.04)                                  7          4079     (28.12)     ±48                                                                              (0.33)                                  28         4779     (32.95)     ±210                                                                             (1.44)                                  ______________________________________                                    

EXAMPLE V

The same steps as in Example IV were repeated, except that raw feed wasreduced so that all of it passed a 50 mesh screen (Tyler) as was thepractice in Example II and was in the form of an about 30% moisturecontent slurry and the amount of fluosilicic acid was 0.25% by weightbased upon the weight of the dry solids in the raw material. The soformed clinker was analyzed using the wet (ASTM C114-69) method, and thepotential compound composition was calculated using the Bogue equationsand these values are set forth in Table VII. In addition, compressivestrengths of a sample of clinker containing 5% by weight gypsum were runin accordance with ASTM C109-73, and they are set forth in Table VIII.As is seen, mortars made from clinker fluxed with a fluorine containingmineral acid achieve compressive strengths in excess of those requiredby ASTM C150-74.

                  Table VII                                                       ______________________________________                                        Analysis of Fluxed Clinker for                                                Raw Feed A Containing 0.25% by Weight Flux                                    ______________________________________                                         Actual Composition (% by Weight)                                             Ingredient                                                                             Wet Analysis                                                                              X-Ray Diffraction Analysis                               ______________________________________                                        SiO.sub.2                                                                              21.76                                                                Al.sub.2 O.sub.3                                                                       5.00                                                                 Fe.sub.2 O.sub.3                                                                       2.20                                                                 CaO      66.10                                                                MgO      3.03                                                                 Na.sub.2 O                                                                             0.18                                                                 K.sub.2 O                                                                              0.68                                                                 FeO      0.02                                                                 SO.sub.3 0.77                                                                 Sulfide S                                                                              0.006                                                                Free CaO 1.97        2.5                                                      LOI      0.37                                                                 Potential Clinker Composition (% by Weight)                                   C.sub.3 S                                                                              64.8        61                                                       C.sub.2 S                                                                              13.5                                                                 C.sub.3 A                                                                              9.5                                                                  C.sub.4 AF                                                                             6.7                                                                  ______________________________________                                    

                  Table VIII                                                      ______________________________________                                        Test Time  Compressive Strength                                                                           Standard Devia-                                   (days)     psi (MPa)        tion psi (MPa)                                    ______________________________________                                        1          1563     (10.77)     ±14                                                                              (0.10)                                  7          4504     (31.05)     ±47                                                                              (0.32)                                  28         5158     (35.56)     ±139                                                                             (0.95)                                  ______________________________________                                    

EXAMPLE VI

In order to demonstrate the wide variety of fluorine containing acidswhich can be employed as fluxing agents in accordance with the teachingsof the present invention, samples were made using the raw feed A ofExample II, reconstituted to a slurry by the addition of water in anamount about 30% by weight, and various amounts of five differentfluorine containing mineral acid fluxing agents, mostly in the form ofacid solutions. The percentage by weight of the acid in the solution isindicated parenthetically. The acids used were those which arecommercially available.

The samples were burned for one hour at various temperatures in platinumdishes, along with control samples in which no flux was added. Theresulting clinkers were analyzed by X-ray diffraction to determine theiralite and free lime contents. The results which are set forth in TableIX (the percentages are by weight), clearly demonstrate the usefulnessof a wide variety of fluorine containing acids as fluxing agents.

                                      Table IX                                    __________________________________________________________________________    Free Lime and Alite Contents of Raw Feed A and                                Flux Burned 1 Hour at Various Temperatures                                    __________________________________________________________________________             Amount                                                                              Free CaO(%)                                                    Flux     (%)   1200° C.                                                                     1250° C.                                                                     1300° C.                                                                     1350° C.                              __________________________________________________________________________    H.sub.2 TiF.sub.6 (60%)                                                                0.25  8.4   7.6   2.3   1.4                                                   0.50  5.4   3.8   0.9   0.5                                          HBF.sub.4 (48%)                                                                        0.25  14.1  11.7  4.3   2.5                                                   0.50  11.2  6.6   1.2   2.0                                          HPO.sub.2 F.sub.2 (100%)                                                               0.25  13.5  11.1  3.4   1.9                                                   0.50  9.2   7.0   2.8   2.9                                          HPF.sub.6 (60%)*                                                                       0.25  2.9   1.0   1.2   0.7                                                         16.9  --    4.3   --                                                    0.50  6.7   4.0   1.5   1.3                                                         9.6   --    1.3   --                                           H.sub.2 SiF.sub.6 (24%)                                                                0.25  9.2   8.1   3.0   1.5                                                   0.50  8.6   4.3   1.1   1.2                                          None     0.    18.9  12.8  7.1   4.5                                                   0.    20.5  16.1  6.2   5.3                                          __________________________________________________________________________                   Alite(%)                                                                      1200° C.                                                                     1250° C.                                                                     1300° C.                                                                     1350° C.                              __________________________________________________________________________    H.sub.2 TiF.sub.6 (60%)                                                                0.25  31    34    55    60                                                    0.50  42    52    59    67                                           HBF.sub.4 (48%)                                                                        0.25  16    22    51    57                                                    0.50  28    40    57    57                                           HPO.sub.2 F.sub.2 (100%)                                                               0.25  17    25    58    56                                                    0.50  31    39    54    57                                           HPF.sub.6 (60%)*                                                                       0.25  45    39    66    64                                                          15    --    53    --                                                    0.50  36    51    57    61                                                          34    --    56    --                                           H.sub.2 SiF.sub.6 (24%)                                                                0.25  31    32    58    61                                                    0.50  36    46    61    60                                           None     0.    3     26    53    57                                                    0.    8     22    54    57                                           __________________________________________________________________________     *The initial data generated (which is also the first occurring) using         HPF.sub.6 appeared to be in error and so a second run with a fresh sample     at 1200° C. and 1300° C., but not 1250° C. and           1350° C., was run to verify the initial numbers. While the second      run was higher, it was still less than control, but since the retest did      not show the initial data to be erroneous, both sets of data are included                                                                              

EXAMPLE VII

To samples of the raw feed A of Example II, reconstituted to a slurry bythe addition of about 30% by weight water, were added varying amounts offour fluorine containing acids in the form of acid solutions, with theamount of acid indicated parenthetically, and four of the calcium saltsof such acids in the form of solid powders of which CaF₂ and CaSiF₆ .2H₂ O are known as fluxing agents. These mixtures, as well as controlsamples in which no flux was added, were then burned in platinum dishesfor one hour at 1200° C. and at 1300° C. The resulting clinkers werethen analyzed, by X-ray analysis, for free lime and alite contents. Thepercentages, in percent by weight, which are set forth in Table X,demonstrate the unexpected high levels of fluxing that the fluorinecontaining acids achieve over their salts, and in fact most of the acidsproved superior to CaF₂.

                                      Table X                                     __________________________________________________________________________    Free Lime and Alite Contents of Raw Feed A With 0.25%                         Flux Added to a Slurry and Burned 1 Hour at Various Temperatures                       Amount                                                                              Free Lime (%)                                                                             Alite (%)                                          Flux     (%)   1200° C.                                                                     1300° C.                                                                     1200° C.                                                                     1300° C.                              __________________________________________________________________________    HF(49%)  0.25  11.2  2.9   33    61                                           CaF.sub.2                                                                              0.25  15.4  3.8   22    55                                           H.sub.2 TiF.sub.6 (60%)                                                                0.25  10.6  2.8   34    59                                           CaTiF.sub.6                                                                            0.25  13.6  3.4   21    58                                           H.sub.2 SiF.sub.6 (24%)                                                                0.25  12.1  3.2   32    58                                           CaSiF.sub.6 . 2H.sub.2 O                                                               0.25  16.4  3.4   20    56                                           HBF.sub.4 (48%)                                                                        0.25  14.8  3.6   19    57                                           Ca(BF.sub.4).sub.2 . H.sub.2 O                                                         0.25  17.0  4.7   13    56                                           None     0.    20.2  7.8    2    44                                           __________________________________________________________________________

EXAMPLE VIII

To samples of the raw feed A of Example II, wherein the particles over50 mesh were reduced to less than 50 mesh and reblended in,reconstituted to a slurry by the addition of about 30% by weight water,were added 0.25% by weight, based upon the weight of the dry solids inthe raw feed, of two known prior art fluxing agents, namely fluorspar(CaF₂) and calcium fluosilicate (CaSiF₆ . 2H₂ O), and a fluorinecontaining mineral acid fluxing agent in accordance with the teachingsof the present invention, namely fluosilicic acid (H₂ SiF₆). Thesemixes, as well as control samples to which no flux was added, were thenburned in platinum dishes for one hour at various temperatures between1300° C. and 1450° C. The particular kiln employed held six samples, andso duplicate samples were run containing fluorspar and calciumfluosilicate. The resulting clinkers were then analyzed for % free limeand % alite contents using the aforementioned X-ray diffraction analysistechnique. The percentages, which are set forth in Table XI and areexpressed as percent by weight, demonstrate the unexpectedly high levelsof fluxing which the fluorine containing mineral acids achieve incomparison to known prior art fluxing agents.

                                      Table XI                                    __________________________________________________________________________    Free Lime and Alite Contents of Raw Feed A, which was Burned                  for 1 Hour at various temperatures and to which was added 0.25%               of two prior art fluxes and one flux in accordance with the present           invention                                                                     __________________________________________________________________________                    Free Lime (%)                                                 Flux    Amount (%)                                                                            1300° C.                                                                     1350° C.                                                                     1400° C.                                                                     1450° C.                             __________________________________________________________________________    None    0       8.3   4.6   3.7   1.9                                         CaF.sub.2                                                                             0.25    5.2   4.0   3.4   2.7                                         "       0.25    4.6   4.2   3.2   2.4                                         CaSiF.sub.6 . 2H.sub.2 O                                                              0.25    5.7   3.6   3.7   1.3                                         "       0.25    5.1   3.2   3.1   1.2                                         H.sub.2 SiF.sub.6                                                                     0.25    3.5   2.5   1.9   0.9                                                         Alite (%)                                                                     1300° C.                                                                     1350° C.                                                                     1400° C.                                                                     1450° C.                             __________________________________________________________________________    None    0       41    54    58    64                                          CaF.sub.2                                                                             0.25    54    58    63    67                                          "       0.25    56    59    61    66                                          CaSiF.sub.6 . 2H.sub.2 O                                                              0.25    53    58    61    64                                          "       0.25    54    55    58    66                                          H.sub.2 SiF.sub.6                                                                     0.25    54    59    61    72                                          __________________________________________________________________________

Thus as has been shown, the use of fluorine containing mineral acids inaccordance with the teachings of the present invention facilitates theburning of hard to burn raw portland type cement mixes, as well asreducing the amount of energy necessary to burn raw feed mixes toproduce portland type cement clinker. When the fluorine containingmineral acid is used in the preferred amount of between about 0.2% to0.5% by weight based upon the weight of the dry solids in the raw feed,mixed using the wet mixing with a raw feed which has been reduced topass 50 mesh (Tyler), and burned at about 1300° C. for between about 1and 2 hours, it is expected that the maximum combination of benefitswill be achieved. That is, the cement clinker produced will be achievedwith a reduced energy consumption while containing high amounts of aliteand low amounts of free lime.

It is well understood by those skilled in the art that certaindifferences are to be expected in the scale-up of the cement makingprocess from the laboratory to the industrial level. Such differenceswill include the residence time of the raw feed within the burning zoneof the kiln and the actual temperature required under such conditions toachieve the beneficial results taught by the subject invention. Thus,while there are differences, it is expected that a scale-up can be doneby those of ordinary skill in the art.

The precise results in scaling-up are difficult to predict since, as iswell known in the art, the burning of a raw feed in a cement kiln is anextremely imprecise process. The temperature in the burning zone canfluctuate as much as 100° C. and the composition and fineness of the rawfeed is not controlled with scientific exactness. Therefore, the preciseamount of fluorine containing mineral acid to be employed may need to beadjusted within the discussed range according to the specific kilnconditions. Further, the amount of fluorine containing mineral acidcould be employed in an amount of up to about 3.0% by weight based uponthe weight of the dry solids in the raw feed since fluorine containingcompounds, usually CaF₂, have been included in cement clinkers inamounts up to 3.0% by weight. Amounts in excess of 3.0% by weight willbegin to produce adverse effects on the compressive strengths of theresulting cements.

While the invention has been described with reference to certainpreferred embodiments thereof, those skilled in the art will appreciatethat various changes and modifications and substitutions can be madewithout departing from the spirit of the invention. It is intended,therefore, that the invention will be limited only by the scope of theclaims which follow.

What we claim is:
 1. In the process for producing portland cementclinker wherein argillaceous and calcareous type materials are groundand blended to form a generally homogeneous mixture and subsequentlyburned in a kiln at a temperature in the range of about 1200° C. toabout 1500° C. to form cement clinker, the improvement comprisingincorporating into said mixture a fluorine containing mineral acidselected from the group consisting of fluosilicic acid, fluotitanicacid, fluoboric acid, and fluophosphoric acid as a fluxing andmineralizing agent in an amount of up to 3.0% by weight based upon theweight of the dry solids in said mixture.
 2. A process according toclaim 1 wherein said acid comprises fluosilicic or fluotitanic acid. 3.A process according to claim 1 wherein said acid comprises fluosilicicacid.
 4. A process according to claim 3 wherein said fluosilicic acid isin the form of a fluosilicic acid solution which comprises about 25% byweight H₂ SiF₆.
 5. A process according to claim 1 wherein said acidcomprises fluotitanic acid.
 6. A process according to claim 5 whereinsaid fluotitanic acid is in the form of a fluotitanic acid solutionwhich comprises 60% by weight H₂ TiF₆.
 7. A process according to claim 1wherein said fluorine containing acid is present in an amount of betweenabout 0.1% and about 1.0% by weight based upon the weight of the drysolids in said mixture.
 8. A process according to claim 1 wherein saidacid is present in an amount of between about 0.2% and 0.5% by weightbased upon the weight of the dry solids in said mixture.
 9. A processaccording to claim 1 wherein said mixture comprises a slurry and saidfluorine containing mineral acid is added to the slurry as an acidsolution.